ST. LOUIS — There's a new extrasolar planet
on the block: a mini-orb likely covered with a deep ocean. And it takes the
record for the lowest mass exoplanet to orbit a normal star, astrophysicists
announced today.
The li'l
planet — weighing in at three times Earth's mass — grabs
the lightweight title from a five Earth-mass planet just announced in
April.
The
super-Earth is called MOA-2007-BLG-192Lb, after its host star
MOA-2007-BLG-192L, which is located about 3,000 light-years from Earth. (A
light-year is the distance light travels in one year, or about 5.88 trillion
miles — 9.46 trillion kilometers.)
The host
star's mass is estimated to be roughly 6 percent of
the sun's mass, or just below the mass needed to sustain nuclear reactions in
its core, thus making it a brown dwarf. Measurement uncertainty means the host mass
could be slightly above 8 percent of a solar mass, which would make MOA-2007-BLG-192L a
very low-mass hydrogen-burning star. The researchers suspect the star is indeed a brown dwarf.
"Our
discovery indicates that that even the lowest mass stars can host
planets," said lead researcher David Bennett of the University of Notre
Dame. "No planets have previously been found to orbit stars with masses
less than about 20 percent of that of the sun, but this finding suggests that
we should expect very low-mass stars near the sun to have planets with a mass
similar to that of the Earth."
Bennett
announced the discovery here at a meeting of the American Astronomical Society
(AAS).
The planet
orbits its host star at about the same distance as Venus orbits the sun. But
the new planet's host star is likely between 3,000 and 1 million times fainter
than the sun, so the top of the planet's atmosphere is probably colder than
Pluto.
The
astrophysicists suggest the tiny planet supports a thick atmosphere,
which along with possible interior heating by radioactive decay, could make the
surface as balmy as that of Earth. (And theory suggests the surface may be
completely covered by a deep ocean.)
The
star-planet system was found using a technique called gravitational
microlensing, in which light from the planet is bent and magnified by the
gravity of a foreground object, such as a star. It marks the seventh planet to date discovered using this method.
"This discovery demonstrates the sensitivity of the microlensing method for finding low-mass planets, and
we are hoping to discover the first Earth-mass planet in the near future," Bennett said.
Most of the nearly 300 exoplanets identified to date have been discovered using the radial velocity method in which
astronomers look for slight wobbles in a star's motion due to the gravitational tug of an orbiting planet. This stellar
wobble technique has found mainly large, Jupiter-like planets or smaller planets that orbit too close to their host stars to harbor life.
The research, funded by the National Science Foundation and NASA, will be detailed in the Sept. 1 issue of Astrophysical Journal.
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